This calculation is normally carried out for each flight phase (climb, cruising, descent) by resolving the numerous constraints that limit the speed, the acceleration or the altitude that are possible for the aeroplane and by setting as an objective the flight parameters to be reached at the end of the phase. In particular, when there are numerous waypoints, it is impossible to use an algorithm running in real time to determine the order in which the constraints must be applied. This order must therefore be selected arbitrarily, which necessarily leads to errors. On the other hand, it is not possible to systematically limit the prediction horizon to a flight leg, since one of the objectives of the calculations is to determine the appropriate transitions for passing from one leg to another taking into account the characteristics of the subsequent legs.